C#操作txt问件，进行清空添加操作

     //把txt清空
            FileStream stream = File.Open(Adr,FileMode.OpenOrCreate,FileAccess.Write);
            stream.Seek(0, SeekOrigin.Begin);
            stream.SetLength(0);
            stream.Close();
            //向txt里面追加信息
            StreamWriter sw = new StreamWriter(Adr,true,Encoding.GetEncoding("gb2312"));
            sw.WriteLine(str);
            sw.Flush();
            sw.Close();